1. Field of the Invention
The present invention relates to a hydrogen generation system configured to reform a fuel comprising compound containing at least carbon and hydrogen as major component to generate a hydrogen gas. More particularly, the present invention relates to a hydrogen generation system that operates uniquely in start, and a fuel cell system having the same.
2. Related Art
One example of a method of generating a hydrogen-rich gas is a steam reforming method in which an organic compound based fuel and water are reacted with each other using a reforming catalyst with heat externally applied. In the steam reforming method, in order to react the water with the fuel, the water needs to exist as steam in the reforming catalyst.
In a plant-scale hydrogen generation system using the steam reforming method, typically, a steam supply portion comprised of a boiler or the like is provided outside of the system, and the steam generated in the steam supply portion and the fuel are supplied to are forming catalyst bed. On the other hand, in a small-scale hydrogen generation system, typically, a steam supply portion is provided inside of the system and steam generated in the steam supply portion is used to cause steam reforming reaction to proceed. In a phosphoric acid fuel cell power generation system using a hydrogen gas as a fuel, which is one type of a distributed power generation system, steam is generated using heat resulting from operation of a fuel cell at a temperature of 200 to 250° C., and is supplied to the hydrogen generation system.
During steady operation of the hydrogen generation system, since the reforming catalyst is subjected to a constant thermal load, change in its catalytic activity is easy to check. On the other hand, during start of the system, the thermal load varies with an elapse of time, which might significantly degrades the catalytic activity. Accordingly, for the purpose of protecting the catalytic activity of the reforming catalyst to be used in steam reforming, it is desirable to supply the steam to the reforming catalyst bed in advance. To this end, in the hydrogen generation system having the steam supply portion such as the boiler, the steam is first generated stably in the steam supply portion and an operation of a reformer of the system is then started. Meanwhile, in a fuel cell power generation system which is one type of the distributed power generation system disclosed in Japanese Laid-Open Patent Application Publication No. Hei. 5-275103, the system is internally provided with a steam supply means, and an operation of a reformer of the system is started while supplying steam generated in the steam supply means to the reformer, using a nitrogen gas.
However, in the small-scale hydrogen generation system, for example, a home cogeneration system using a polymer fuel cell, if the hydrogen generation system is used along with a steam supply unit using an external heat source such as the boiler, energy efficiency, an operating cost, and cost of the system are reduced. Since the small-scale hydrogen generation system has a heat source for supplying heat required for reforming reaction, the heat derived from the heat source is commonly used to generate the steam. For example, in the hydrogen generation system using a natural gas as a fuel, steam reforming reaction is adapted to proceed in a reforming catalyst bed at a temperature of 650 to 750° C. Also, in the system using another hydrocarbon based fuel, the reforming catalyst bed is heated to a temperature approximately equal to that in the system using the natural gas. For the purpose of efficiently utilizing heat energy resulting from reforming reaction conducted at this high temperature, a hydrogen generation portion is generally configured to use heat resulting from the reforming reaction to generate steam. Thus, the hydrogen generation system provided with the steam generation portion allows effective use of the heat energy resulting from the reforming reaction during steady operation.
The hydrogen generation system for use in the home system needs to be adapted to operating conditions including frequent system starting and stopping operations, in contrast to the large-scale (e.g., plant-scale) system. During start of the system, heat energy supplied from the heat source is first used to heat the reforming catalyst and is then used to generate steam in the steam generation portion. Thus, since the heat energy is used with priority to heat the reforming catalyst during start of the system, there is a possibility that sufficient steam is not generated in the steam generation portion and is not supplied to the reforming catalyst at that time. As a result, depending on a configuration of the system or heating condition, the reforming catalyst is heated excessively and is subjected to large thermal load, and thereby the catalytic activity of the catalyst is degraded.